How does adaptation actually work?

One of the major questions I address in Purpose and Desire is how adaptation really works. More importantly, what role does adaptation play in the evolutionary process?

Modern Darwinism has a deeply ambivalent relationship to adaptation. On the one hand, it seems so vital to the Darwinian story—natural selection has produced all the wonderful and ingenious contrivances of survival, so the story goes. On the other hand, adaptation is a deeply intentional and purposeful phenomenon, and this runs contrary to Darwin’s own aim to treat evolution as a machine, devoid of problematic intentionality.

The two positions cannot really be reconciled, so the result is one of those grand incoherencies that permeate modern Darwinism: adaptation is the outcome of selection of “apt function” genes, with “apt function” genes defined as the products of selection. This, in turn, has led modern evolutionary biology into another grand incoherency: that adaptation in organisms (physiological adaptation) has no relevance to adaptation in evolving lineages (evolutionary adaptation). In other words, how life actually works has nothing to say about how life actually evolves. That’s a seriously-held position. Nevertheless, it can stand only because there is not a serious alternative to the adaptation-as-selection-of-adaptation-genes tautology.

Yet, alternatives are emerging, piecemeal. Take what is probably the most primitive cell function (Schultz 1989): management of the balance of ions like sodium, potassium and chloride across the cell membrane.

Cells maintain significant ion imbalances across their membranes: high potassium and low sodium inside, and low potassium and high sodium outside. The cell does this by pumping both ions across the membrane (ATPase in the above diagram), which offsets the losses across the sodium (Na+ and K+) channels. It’s been compared to trying to keep a leaky bucket full by pouring in water. In fact, the primitive function here is probably to manage flows of water in the cell: water tends to follow these ion imbalances.

How hard a cell has to work to do this will depend upon the environment. The cell will have to work differently in sea water, compared to fresh water, for example.

In a standard Darwinian model, the adaptation of a cell to these environmental challenges will be selection of the genes that specify the pumps (ATPase) and the channels. Going from sea water to fresh water, for example, will require a harder-working pump, and channels that may be less “leaky” than in sea water. To go from sea water to fresh water will require a long process of genetic selection. In every generation of cells, there will be genetically-specified variation in these components’ properties, and natural selection will, generation-over-generation, eliminate the “inapt” function genes, and favor the “apt function” genes.

Except that’s not how it seems actually to work. How these components work adaptively is almost completely divorced from the genetic specification of functional properties. Some recent work on sodium transport (Razavi et al. 2017) illustrates this nicely.

The work concerns a membrane protein known as the dopamine transporter. It functions like the ATPase in the diagram above, that is to say, it helps translocate sodium ions across the cell membrane. The conventional model for how this works is that the protein grabs a sodium on one side of the membrane, then physically translocates it to the other side of the membrane, kind of like a lazy-susan can be used to pass the salt from one side of the table to the other.

It turns out that’s too simplistic a model. What really seems to happen is this. Sodium ions from one side of the membrane are passed through the dopamine transporter by being handed off from one amino acid residue to the next, until it’s eventually released on the other side. Now, here’s the rub: there are more than a dozen likely pathways a sodium ion can follow. And which pathway it follows depends upon the protein’s hydration: its interaction with water.

In other words, variation and adaptability are built into the system right from the get-go. Genetic variation in the structure and function of the proteins is not required. Genetic variation doesn’t even enter into it. This means that adaptation can work in the absence of natural selection. However, whether the cell survives to reproduce or persist—is strictly a physiological phenomenon, driven by homeostasis.

References

Schultz, S. G. (1989). Volume preservation: Then and now. News in Physiological Science
4 (October 1989): 169-172.

Razavi, A. M., G. Khelashvili, et al. (2017). A Markov State-based Quantitative Kinetic Model of Sodium Release from the Dopamine Transporter. Scientific Reports
7: 40076.

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